Sure the DS will shot in the middle of their own formation to take out some Rebels. But there are even better way to destroy the Rebel Fleet: detonate the core of the Death Star, that would surely do the business.
That was obviously what Lando was thinking. The fleet needs to stay in the fight in order to buy time for Han to take down the shield. They can either take their chances with the Star Destoyers, the Death Star, or both. They decide to close with the Star Destroyers on the chance that the DS would not be able to continue firing at them but at least if it did they "might just take a few of them with us!" They chose correctly.
No Lando meant what every sane person would think: the DS was defended by an unpenetrable shield so they can do nothing against it, the same does not holds for the star destroyers. They can be knocked out, so the have a chance for a fight.
The decision to close to point blank range was not made when presented with the decision to fight Star Destroyers or an impenetrable shield. And remember also that Akbar didn't know that he was going to have to fight Star Destroyers, the Imperial fleet snuck up on and "trapped" him. Trapped implies he couldn't get away, and was FORCED to fight Star Destroyers not chose to.
The decsion to close to pont blank range was made when presented with the choice of fighting an operational DS or retreat. The prospect of fighting a winning battle with Star Destroyers was never even mentioned in the decision making process anywhere in the films. The closed with the Star Destoyers for the sole purpose of preventing the DS from obliterating them outright and thus buying more time for the shield to be dropped. Just because Star Destroyers might take more time to destroy the Rebel fleet than the DS does not mean that the Rebel fleet ever had a reasonable chance against the Star Destoyers. It was a calculated decision on who would kill them faster, the rebel capital ships being nothing more than expendable cover so the fighters could survive as long as possible.
This conversation I explain above is EXACTLY the one that Lando and Ackbar have, they simply don't have to parse semantics for posters who can't accept the reality that the Rebels were hopelessly outgunned by either Imperial force.